1. Field of the Invention
The present invention relates to semiconductor switching circuits for interrupting and switching high frequency signals in the VHF and UHF bands, and it also relates to semiconductor devices using the semiconductor switching circuits.
2. Description of the Related Art
Various kinds of wireless devices operating in the ultrahigh frequency range incorporate semiconductor switching circuits to interrupt and switch transmitted high frequency signals. In order to reduce power consumption, such switching circuits use metal semiconductor field effect transistors (MESFET) formed of a GaAs compound semiconductor or the like.
FIG. 9 shows an example of a semiconductor switching circuit of the above-mentioned type. As shown in the figure, in the semiconductor switching circuit, between an input terminal 71 for inputting a high frequency signal and an output terminal 72 outputting the high frequency signal, there is arranged a first field effect transistor (hereinafter referred to as a first FET) 77 for opening and closing the input/output terminals. The drain of the first FET 77 is connected to the input terminal 71 and the source thereof is connected to the output terminal 72. The gate of the first FET 77 is connected to a switching terminal 73 via a resistor 81. The switching terminal 73 receives a switching signal for controlling the first FET 77.
Between the input terminal 71 and a ground potential terminal 76 there is arranged a second field effect transistor (hereinafter referred to as a second FET) 78 for obtaining isolation characteristics. The drain of the second FET 78 is connected to the input terminal 71 and the source thereof is connected to the ground potential terminal 76. The gate of the second FET 78 is connected to a switching terminal 74 via a resistor 82. The switching terminal 74 receives a switching signal for controlling the second FET 78. The first FET 77 and the second FET 78 are MESFETs.
In the above arrangement, when the first FET 77 and the second FET 78 are depletion-type N-channel FETs, the FETs 77 and 78 are driven by applying a positive voltage. The source of the second FET 78 is connected to a terminal (external bias terminal) 75 via a resistor 83. A positive bias voltage is applied to the terminal 75. As a result, in the semiconductor switching circuit shown in FIG. 9, by applying either a positive switching voltage higher than a predetermined threshold voltage or a ground potential to the FETs 77 and 78 from the switching terminals 73 and 74 which receive the switching signals, the circuit between the input/output terminals 71 and 72 can be opened and closed.
For example, with constant bias voltage on the terminal 75, when the same voltage is applied to the terminal 74, the FET 78 becomes ON; and when ground potential is applied to the terminal 74, the FET 78 becomes OFF.
Thus, by applying appropriate switching voltages to the terminals 73 and 74, it can be arranged that when the first FET 77 is conducting, the second FET 78 is not conducting; and when the first FET 77 is not conducting, the second FET 78 is conducting. By operating the second FET 78 in this way, sufficient isolation characteristics between the input terminal 71 and the output terminal 72 can be maintained, particularly when the first FET 77 is not conducting.
Between the source of the second FET 78 and the ground potential terminal 76, a parasitic inductance component 85 is generated by a bonding wire and a lead frame, when the semiconductor switching circuit is formed into an IC chip to be used as a semiconductor device. In this case, in terms of the parasitic inductance 85, the higher the frequency, the higher the impedance. Thus, since the impedance between the second FET 78 and the ground potential terminal 76 becomes higher in a high frequency region, the impedance of the input terminal cannot be sufficiently lowered. As a result, when the first FET 77 is not conducting and the second FET 78 is conducting, satisfactory isolation characteristics between the input terminal 71 and the output terminal 72 cannot be maintained.
Therefore, in this semiconductor switching circuit, in order to obtain sufficient isolation characteristics between the input terminal 71 and the output terminal 72, a capacitance element 84 is connected in series with the parasitic inductance component 85. In other words, the capacitance element 84 has a value set to permit serial resonance with the parasitic inductance 85 at a specified frequency. In this case, a resonance frequency necessary to improve the isolation characteristics between the input terminal 71 and the output terminal 72 is represented by the symbol f, the value of the inductance component 85 is represented by the symbol L, and the value of the capacitance element 84 is represented by the symbol C. A condition for producing the serial resonance is represented by C=1/(4xcfx802xc3x97f2xc3x97L). When the value C of the capacitance element 84 is determined and thereby a serial resonance is produced at a specified frequency, the impedance between the input terminal 71 and the ground potential terminal 76 can be minimized. Accordingly, when the first FET 77 is not conducting and the second FET 78 is conducting, good isolation characteristics between the input terminal 71 and the output terminal 72 can be maintained. In addition, besides the above function, the capacitance element 84 has a DC blocking function that isolates the power supply voltage applied to the external bias terminal 75 from the ground potential terminal 76.
The capacitance element 84 is generally formed as a metal-insulation capacitor on the semiconductor chip. After the capacitance clement 84 and the FET have been integrated into a chip to form a monolithic microwave integrated circuit (hereinafter referred to as MMIC), the value of the parasitic inductance 85 generated by the bonding wire and the lead frame can no longer be adjusted. Thus, it requires a lot of time and experimentation to set the value of the capacitance element 84 most appropriately.
In addition, the capacitance element 84 is formed not by a pure capacitance component but by a capacitance component including a parasitic inductance component generated by metal electrodes, wires and the like. Consequently, since an inductance component required for the serial-resonance condition is equivalent to a sum of the inductance components of the parasitic inductance 85 and the capacitance element 84, the configuration of the metal wire used needs to be considered when setting the value of the capacitance element 84.
The entire inductance component, which is equivalent to the sum of the inductance components of the parasitic inductance 85 and the capacitance element 84 generated by the bonding wire and the lead frame, usually has a small value of a few nH or lower. Therefore, in order to produce a serial resonance in a low frequency region, a large capacitance component relative to this small inductance is required. When there is provided a large capacitance component, changes in the impedance near a frequency at which the impedance of the serial resonance circuit is zero become smaller. Thus, a frequency band in which the impedance of the serial resonance circuit is small is broadened with respect to the resonance frequency, and therefore, sufficient isolation can be provided over a wide frequency range. In contrast, in order to produce a serial resonance at high frequencies, a small capacitance component relative to the inductance is required. In this situation, near the frequency at which the impedance of a serial resonance circuit is zero, the impedance changes increase. As a result, the frequency band in which the impedance of the serial resonance circuit is small is narrowed, which greatly narrows the frequency band where sufficient isolation is obtainable.
Specifically, for example, in a case in which the inductance L of a serial resonance circuit is 1 nH, when the resonance frequency f is 800 MHz, the capacitance C is approximately 39.6 pF, and when the resonance frequency f is 5 GHz, the capacitance C is approximately 1 pF. In this situation, a frequency band in which the impedance Z of the serial resonance circuit is 1 xcexa9 or lower can be obtained by the following quadratic equation, in which f represents the resonance frequency:
2xcfx80Lf2xe2x88x92fxe2x88x921/(2xcfx80C)=0,
which is obtained by modifying the equation Z=2xcfx80Lfxe2x88x921/(2xcfx80fC). Based on this quadratic equation, when the resonance frequency is 800 MHz, the frequency band in which the impedance of serial resonance is 1 xcexa9 or lower is between 724 MHz and 889 MHz. Thus, this circuit has a wide bandwidth, namely 20.6% with respect to the resonance frequency 800 MHz. In contrast, when the resonance frequency is 5 GHz, the frequency band in which the serial-resonance impedance is 1 xcexa9 or lower is between 4.954 GHz and 5.113 GHz. Thus, the obtained band range is only 3.18% with respect to the resonance frequency 5 GHz, which is significantly narrower. As a result, in a high frequency region, sufficient isolation cannot be obtained in many cases.
Furthermore, as shown above, since the capacitance value of the capacitance element 84 is small in the high frequency band, only a small deviation in the capacitance changes the resonance frequency significantly. As a result, it is extremely difficult to adjust the capacitance value most appropriately.
Accordingly, the present invention provides a semiconductor switching circuit and a semiconductor device that can provide sufficient isolation characteristics at high frequencies.
The present invention provides a semiconductor switching circuit and a semiconductor device including a first semiconductor switching element connected between a first terminal and a second terminal, a second semiconductor switching element, one end of the second switching element being connected to one of the first and second terminals, and an open stub connected to the other end of the second switching element.
The open stub connected to the other end of the second semiconductor switching element may be a distributed-constant element. Thus, when the electric length of the open stub is equal to an odd multiple of xcex/4 with respect to a predetermined frequency wavelength xcex, the impedance of the open stub is zero. Additionally, since the open stub is a distributed-constant element, as compared with a lumped-constant element producing a serial resonance by the combination of a capacitance and an inductance, impedance changes are usually small in a frequency band near the frequency at which the impedance of the stub is zero. In other words, in a high frequency band, in the case of the lumped-constant element producing a serial resonance, due to the influence of the inductance value of the lumped-constant element, in the frequency region where the impedance value of the serial resonance is small, the impedance changes steeply and the frequency band in which the serial-resonance impedance is small is narrowed. On the other hand, with a distributed-constant element, since the open stub is not connected to a bonding wire and a lead frame which would generate inductance components, there is no influence of inductance components on the open stub, so that a low impedance can be obtained over a sufficiently wide frequency range.
In addition, even when the open stub is connected to a bonding wire and a lead frame which generate inductance components, the inductance component hardly narrows the low-impedance frequency band of the open stub, although the small inductance component increases the electric length of the open stub.
In addition, in the high frequency range, since the open stub has a wide low-impedance frequency band, it is simple to set and adjust the electric length of the open stub.
Therefore, even when a high frequency signal is input to the input terminal, when the first semiconductor switching element is not conducting, the second semiconductor switching element can be brought into conduction with a low impedance. As a result, satisfactory isolation characteristics can be obtained.